The oil lobby paid Edelman $327.4m for lobbying and public relations, according to an investigation by the Center for Public Integrity. Those earnings, which include money later spent by Edelman for advertising, cover only a five-year period from 2008-2012.

That relationship was by no means exclusive. API paid another PR firm, FleishmanHillard, an additional $51m.

But Edelman had favoured status, according to the Climate Investigations Center, which has tracked the company’s complicated relationship with the fossil fuel industry. In 2008, the oil lobby paid Edelman $75m, more than a third of the $203m in revenues collected in membership dues from ExxonMobil, Chevron and other oil companies.

Climate changeable: waffling lands PR firm Edelman in hot water

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The lucrative relationship was not without costs. Over the past year, Edelman came under growing public pressure for its ties to fossil fuel companies and industry groups which have promoted misinformation about climate change.

Last year, Edelman was caught out when other major public relations firms announced they would no longer work for climate deniers, in response to a Guardian report.

The company also faced scrutiny for advising TransCanada pipeline company to run a “perpetual campaign” against opponents of a pipeline project across eastern Canada. TransCanada later announced it had dropped Edelman.

Such hardball tactics – and the accusations of climate denial – put Edelman in an uncomfortable position with some of its other clients, according to Kert Davies of the Climate Investigations Center.

API does not explicitly deny climate change, but its website suggests – incorrectly – that there is some doubt whether burning of fossil fuels is warming the planet.